1. Field of the Invention
The invention pertains generally to liquid crystal cells and, more particularly, to a matrix addressed liquid crystal optical display.
2. Art Background
Liquid crystals are liquids whose molecules display ordering. This ordering is characterized by a localized alignment of the liquid crystal molecules. The direction of this localized alignment and thus the spatial orientation of the liquid crystal molecules can be changed by the application of electric fields to produce corresponding changes in the optical properties of the liquid crystal. For example, a change in the spatial orientation of a liquid crystal produced by the application of an electric field affects the polarization of light, e.g., visible light in the 4500 to 8000 Angstrom range, incident on the liquid crystal. This change in the polarization of incident visible light can be perceived, for example, by viewing the liquid crystal between a polarizer and analyzer. That is, the change in the polarization of the incident light will result in a change in the amount of the light which is transmitted through the liquid crystal when the liquid crystal is arranged between an appropriately oriented polarizer and analyzer.
A device in which a liquid crystal is confined between two bounding surfaces, at least one of which is transparent to light, is called a liquid crystal cell. Typically, two glass plates are used to confine the liquid crystal. In addition, a plurality of electrodes is usually applied to the glass plates in order to subject discrete portions of the liquid crystal, referred to herein as pixels, to electric fields to thereby alter the spatial configurations, and thus the optical transmission properties, of the pixels. Thus, if some pixels are made to transmit incident light while others do not, an optical effect is produced which can be used to display information.
In one particular type of liquid crystal cell, called a liquid crystal twist cell, the liquid crystal molecules exhibit at least two different spatial configurations when subjected to appropriate electric fields. In at least one of these configurations the molecules assume a twisted, helical configuration, with the axis of the helix oriented perpendicularly with respect to the bounding surfaces. On passing through the liquid crystal twist cell, the plane of polarization of plane polarized incident light is rotated by the helical orientation of the molecules. When the liquid crystal molecules are in the second configuration, the different orientation of the molecules has a different effect on the polarization of the incident light. By, for example, placing the liquid crystal twist cell between crossed polarizers, pixels in one or the other spatial configuration will either transmit incident light through the cell or not transmit light, and thus appear to be light or dark.
Liquid crystal cells have been used in relatively small optical displays, having less than about 100 pixels. These relatively small displays include the now common liquid crystal wristwatches. In many of these relatively small displays the spatial orientation of the liquid crystal of each pixel is regulated by voltage pulses transmitted through an individual electrical lead connected to just that pixel of the display, so that each pixel is individually electrically driven. While the use of individually driven leads is considered acceptable in relatively small optical displays, the cost and complexity of using individually driven leads in a large optical information display, consisting of more than about 100 pixels, is presently prohibitive.
Because of their low power consumption and thin profile, liquid crystal cells have also been used as components for flat panel, large information optical displays, consisting of more than about 100 pixels. The optical transmission states of the many pixels of such large displays are not regulated with individually driven leads. Rather, in order to reduce costs and complexity, the pixels of such large arrays are usually formed as the intersections of an array of row and column electrodes, which intersections define a matrix of liquid crystal pixels. In each liquid crystal pixel the optical transmission is switched from one state to another by applying an appropriate voltage to the pixel through the interaction of voltages applied to the row and column electrodes intersecting at the pixel. Selective switching of individual pixels from one optical transmission state to another in such a matrix array, without appreciably affecting other pixels, is referred to as dynamic matrix addressing.
Dynamically matrix addressed, liquid crystal displays which do not exhibit bistability are limited in size, i.e., are limited in the number of addressable rows or columns. The particular size limit is dependent on the type of scheme used to matrix address the display as well as the properties of the display. A liquid crystal cell which exhibits bistability is one which displays memory with respect to two different spatial orientations of the liquid crystal (which correspond to two different optical transmission states of the cell). That is, the liquid crystal of the cell can be switched to a new spatial orientation by applying, for example, a relatively high voltage across the cell, and the liquid crystal remains in the new orientation even if the voltage is entirely removed or reduced to some lower, nonzero value, i.e., a holding voltage. In addition, in a bistable cell the liquid crystal of the cell is switched to its former orientation by, for example, applying a relatively low voltage (lower than the holding voltage) across the cell and the cell remains in this state even after the relatively low voltage is removed or the voltage is returned to the holding voltage value. The pixels of liquid crystal displays which do not exhibit bistability require continual refreshing by an appropriate voltage signal to maintain an optical contrast. In the case of a nonbistable, liquid crystal matrix display which includes a liquid crystal cell responsive to the root-mean-square (rms) value of an applied AC field (which is the case with liquid crystal twist cells), the upper limit on the number of rows or columns of the matrix display is inversely proportional to the square of the ratio of the rms-voltages required to produce an acceptable optical contrast. Thus, an increase in the number of rows or columns of the display may require a corresponding decrease of this ratio (see A. R. Kmetz, in Nonemissive Electrooptic Displays, edited by Kmetz and von Willisen (Plenum, N.Y., 1976), pp. 270-273 ). Because there are practical limitations on the decreases in the ratios of these rms-voltages which are achievable with liquid crystal displays, it then follows that the number of rows or columns of liquid crystal matrix displays which do not exhibit bistability is limited. However, in principle, no such limit exists for liquid crystal matrix displays which do exhibit bistability of states. Consequently, an important objective of those attempting to perfect large information liquid crystal matrix displays has been to fabricate a liquid crystal matrix display which exhibits bistability.
Efforts directed at developing bistable liquid crystal displays have resulted in the development of temporarily bistable liquid crystal cells (see, e.g., E. P. Raynes, in Nonemissive Electrooptic Displays, edited by Kmetz and von Willisen (Plenum, N.Y., 1976), pp. 29-36). A temporarily bistable liquid crystal cell is one which exhibits two states. The cell is switched to one of these states by applying a voltage greater than, or equal to, a threshold voltage across the cell. If the voltage is removed, the cell quickly reverts to the other state, but this reversion can be retarded for a time by applying a biasing voltage lower than the threshold voltage.
Two matrix addressing schemes which have been used with temporarily bistable liquid crystal matrix displays are the "three-to-one" and "two-to-one" matrix addressing schemes. The application of these matrix addressing schemes to temporarily bistable liquid crystal matrix displays has been reviewed by Kmetz in Nonemissive Electrooptic Displays, edited by Kmetz and von Willisen (Plenum, N.Y., 1976), pp. 268-269. Both of these addressing schemes employ biasing voltages to retard the relaxation of the liquid crystal from one state to the other. Because these matrix displays are only temporarily bistable and thus require periodic refreshing voltage signals, these displays are limited in their multiplexing capacities (the number of addressable columns or rows). The upper limits on the multiplexing capacities of these displays differ depending on which addressing scheme, e.g., three-to-one or two-to-one, is employed. In general, it cannot be predicted which addressing scheme is preferable for a particular device. For one particular temporarily bistable device reviewed by Kmetz, supra, p. 269, some improvement in writing speed and multiplexing capacity is obtained, at the cost of higher operating voltages and some flicker, by the use of the two-to-one addressing scheme rather than the three-to-one addressing scheme.
A liquid crystal twist cell which is truly bistable, rather than merely temporarily bistable, has been disclosed in U.S. Pat. No. 4,239,345, issued on Dec. 16, 1980, to D. W. Berreman and W. R. Heffner. The cell is characterized by at least two stable states, either of which is stable as long as no external, threshold energy, e.g., no voltage in excess of some threshold voltage, is applied to the cell. External energy is necessary only for switching the cell between the stable states. The potential inherent in this or any other bistable, liquid crystal twist cell, i.e., of using a truly bistable cell to fabricate a large information optical display which, in principle, is not limited in size, has not yet been realized.